1. Field of the Invention
The present invention relates to a pipe coupling fabricated of shape memory alloy.
2. Description of the Prior Arts
A pipe coupling fabricated of shape memory alloy is disclosed in Japanese Examined Patent Publication No. 4898/79. A plurality of ring protrusions are installed on an internal clamping surface of a cylindrical coupling member of the coupling. Ends of a pipe to be joined to the pipe coupling are inserted into the pipe coupling from both sides of the coupling member. The coupling member contracts in the radial direction by raising a temperature of the coupling member, into which the ends of the pipe have been inserted, to its transition temperature or more and the ring protrusions bite into an external surface of the pipe to be joined to the coupling. In this way, the ends of the pipes are joined to each other.
However, when a pipe is in the following state, a pipe coupling is hard to seal reliably.
a) When the pipe has surface flaws, especially, surface flaws in the axial direction of the pipe.
b) When a seam weld pipe has a small weld reinforcement of a bead and a shape of its toe is not smooth.
c) When a roundness of the pipe is not good.
d) When a tolerance of an outer diameter is large.
Particularly, when a ratio of a wall thickness "t" to a diameter "D" of the pipe is large, a reliable seal property is hard to obtain.
In the above-mentioned case, in order that the ring protrusions can bite well into the external surface of the pipe, it is thought that the shape of the ring protrusion is made to be an acute angle or that a radius of curvature of the pipe is made small. However, even when a hardness of a material of the coupling member is equal to a hardness of the pipe to be joined or slightly over the hardness of the pipe, the ring protrusions themselves are deformed. Consequently, the ring protrusions cannot bite into the external surface of the pipe in order to raise the seal property.